This seems very obvious and I am having a bit of trouble producing a formal proof.
sketch proof that the composition of two polynomials is a polynomial
Let
be two complex polynomials of degree where and .
Now,
which is clearly a complex polynomial of degree .
sketch proof that the composition of two rational functions is a rational function
A rational function is a quotient of polynomials.
Let
Now,
Notice that is a polynomial as
where
are both polynomials. Hence is a rational function as it is the quotient of polynomials.